Antimetabolites targeting the key enzymes of purine de novo synthesis are important drugs used in cancer chemotherapy. A major limiting factor in the clinical effectiveness of antimetabolites is tumor nonresponsiveness due to inherent and acquired resistance. Because cancer cells can circumvent the growth-inhibitory effect of antipurine antimetabolites via the salvage of extracellular preformed purines such as hypoxanthine, nucleobase transporter's that mediate cellular uptake of purines represent a promising target for overcoming tumor resistance and improving the clinical efficacy of antipurine antimetabolites. We hypothesized that a combination of nontoxic and high potency inhibitors of nucleobase transporters with antipurine antimetabolites can block both purine de novo and salvage pathways, leading to enhanced therapeutic afficacy and decreased drug resistance. To date, mammalian nucleobase transporters have not been identified at the molecular level and there are no specific inhibitors available for these transporters. To pave the way for utilizing nucleobase transporters for anticancer therapy, it is essential to isolate genes encoding these transporters from cancer cells and identify specific inhibitors for these transporters. Recently, we developed a complementation cloning approach in yeast that allows us to isolate nucleobase transporter genes from other organisms. In studies proposed under Specific Aim 1, we will use this approach to isolate nucleobase transporter genes from two human cancer cell lines where the presence of distinct nucleobase transporters has been demonstrated. Once clone, we will elucidate the functional characteristics of these transporters and determine their expression in normal tissues and neoplastic cells. In studies proposed under Specific Aim 2, we will use yeast as an expression and assay system to identify small molecule inhibitors for these transporters through both small-scale analysis of suspected compounds and large-scale screening of several compound libraries. These studies will provide a mechanistic understanding of nucleobase transport in cancer cells. Furthermore, it will make molecular and chemical tools available for further developing these transporters as a target for anticancer drug discovery.